The invention relates to a gallium nitride-based device and method. More particularly, the invention relates to a graded In-content quantum well device and method.
A quantum well (QW) is a potential boundary that confines particles to a planar, substantially two-dimensional region. Each layer in a multiple quantum well structure has a very small thickness. The electrons and holes in the layer cannot move freely in the direction of thickness and are substantially confined two-dimensionally in a plane perpendicular to the thickness direction. The two dimensional confinement increases bound energy of Coulombic electron and hole attraction so that excitons occur under heat energy at room temperature.
A QW can be formed as part of a semiconductor by having a material, such as gallium arsenide sandwiched between two layers of a wider bandgap material such as aluminium arsenide. A quantum well effect can be achieved in a device with alternating tens to hundreds of two kinds of very thin semiconductor layers with different band gaps. Such structures can be grown by molecular beam epitaxy (MBE) and chemical vapor deposition (MO-CVD). These procedures can provide a layer down to molecular monolayer size.
Because of a quasi two dimensional nature, electrons in a quantum well have a sharper density of state than bulk materials. As a result, quantum well structures are in wide use in diode lasers. They are also used to make HEMTs (High Electron Mobility Transistors), which are used in low-noise electronics.
Quantum well-based emitters (LEDs and diode lasers) in the blue, green, and red regime are important for solid state lightings and medical applications. These applications require highly efficient blue, green, and red diodes integrated in a single semiconductor chip. However only low efficiency can be attained with typical gallium nitride-based quantum wells such as InGaN-based QWs, particularly as emission wavelength is extended beyond green color into red color.
The GaN-based quantum well semiconductor suffers from two main issues. First is high defect or dislocation density, and second is large charge separation in the quantum well. High defect density can be caused by lattice mismatch strain and immature epitaxy of the nitride-material system leading to very high threading dislocation density, thus this results in high nonradiative efficiency. The large charge separation in quantum well results in low radiative recombination rate and low optical gain.
The drawings illustrate various gallium nitride-based devices. FIG. 8A shows conduction and valence band lineup of a type-II In0.18Ga0.82N—GaN0.98As0.02 QW. The QW hole is confined in the center GaNAs QW layer and the peaks of the electron and hole wavefunctions coincide with one another giving rise to high electron-hole wavefunction overlap Γe-hh˜68.9%. However as shown by FIG. 8B, for In-content beyond 18%, the affect of the polarization-induced electric field is so significant, that steep bending of the valence band edge is caused. As a result of this steep bending, hole confinement is shifted and the hole is now confined in the bottom InGaN QW layer (no longer confined in the center GaNAs layer). This shift in hole confinement leads to spatial separation of the hole and electron wavefunction peaks. This results in detrimental reduction in overlap, Γe-hh down to ˜35.5% in the FIG. B example.
There is a need for a higher performing gallium nitride-based device characterized by a reduced overlap in type II InGaN—GaNAs QWs with greater than 18% In content.